The primary fuselage structure of airliners is mainly composed of skins reinforced by circumferential frames and longitudinal stiffeners.
This primary structure comprises a lower floor structure connected at its lateral ends to the circumferential frames and supported by a lattice of struts, sometimes called rods. The aforementioned lower floor is generally called the cargo floor.
The primary structure also comprises a passenger cabin floor structure connected by its lateral ends to the circumferential frames and also supported by struts. The passenger cabin floor is called the main floor in the following.
FIG. 1 illustrates a typical example of such a primary fuselage structure 10 seen in cross-section, and shows in particular a circumferential frame 12, a beam 14 of the lower floor structure 16, a lattice of struts 18 each having a lower end fixed to the circumferential frame 12 and an upper end fixed to the beam 14 of the lower floor structure 16 so as to support it, as well as a beam 20 of the main floor structure 22, and two struts 24 each having a lower end fixed to the circumferential frame 12 and an upper end fixed to the beam 20 of the main floor structure 22 so as to support it. The lower floor structure 16 makes it possible to support the floor of the baggage hold 26 of the airplane, while the main floor structure 22 makes it possible to support the floor of the passenger cabin 28.
FIG. 2 illustrates a second typical example of a primary fuselage structure 10, viewed in cross-section, which differs from the primary structure of FIG. 1 in that the passenger cabin comprises a lower deck 30 and an upper deck 32, such that the primary structure also comprises an upper floor structure 34 designed to support the floor of the upper deck.
In certain cases of an airplane crash occurring under low speed conditions, particularly as regards the vertical component of that speed, it is desirable that the impact energy associated with that vertical component be optimally dissipated by a lower portion of the primary structure, defined below the main floor structure 22, such that the repercussions of the vertical impact suffered by the passengers in the cabin area remain below regulatory limits.
To this end, it is desirable, during a first phase immediately following a vertical impact, that a first portion 36 of the primary structure, defined under the lower floor or cargo floor, crushes while transmitting a low level of force to a second portion 38 of this primary structure, defined above the lower floor. This first phase, which typically lasts on the order of 150 milliseconds, has the aim of allowing maximum energy absorption by the first portion 36 of the primary structure. This first phase is normally followed by a second phase during which the second portion 38 of the structure deforms while absorbing the surplus energy that the first portion was unable to absorb.
It is therefore desirable to limit the level of the forces transmitted to the second portion 38 of the primary structure by its first portion 36 during a crash, with the purpose of ensuring that the second portion 38 does not begin deforming prematurely, which would limit the crushing of the first portion 36 and hence the quantity of energy absorbed by the entire lower portion of the primary structure. Limiting the force level transmitted to the second portion 38 of the primary structure during the aforementioned first phase is also desirable for reducing the capacity for resisting these forces required of said second portion 38, and thus limiting the mass of the elements constituting this second portion such as the struts 24 supporting the main floor structure and the circumferential frames 12.
However, a large portion of the forces applied to the first portion 36 of the primary structure during a crash are transmitted to the lower floor structure 16 by the struts 18 supporting it. These forces pass through the lateral ends of the lower floor structure and thus reach directly regions of the circumferential frames 12 which are situated at the base of the second portion 38 of the primary structure, causing premature deformation of this second portion.
Reducing the stiffness of the struts 18 supporting the lower floor structure 16 would allow a reduction in the level of the forces transmitted to the second portion 38 of the primary structure in the event of a crash, but this solution is not satisfactory because it necessarily results in a reduction of the permissible load on the lower floor during normal operation of the airplane.
Another solution which was proposed by the applicant is described in international patent application WO 2009/101372 A1. In this document, a floor structure is supported by struts in the form of compression beams made of composite material, oriented in the vertical direction and each fixed to a circumferential frame. These struts each support at their upper end a gusset able to cut the strut under the influence of a high-level compressive force such as that resulting from a crash.
However, this solution requires that each strut be capable of resisting the cutting means of the corresponding gusset when the strut is subjected to a compression force induced by the loading on the floor, in normal operation, so that the cutting of the strut occurs only in the event of a crash.
Thus this solution necessitates overdimensioning of the struts, which leads to an undesirable increase in the overall mass of the primary structure of the fuselage.
In addition, as explained above, the impact energy resulting from a crash can be largely absorbed by the lower portion of the primary structure of the fuselage of the airplanes.
In fuselages having a metal primary structure, made of aluminum for example, absorption of impact energy results from a plastic deformation of the metal elements constituting the primary structure. The metal elements that deform are in particular the skin, the longitudinal stiffeners, the struts 18 supporting the lower floor structure 16, the struts 24 supporting the main floor structure 22, and a lower portion of each circumferential frame 12 defined below the main floor structure.
However, the use of composite materials in airplane construction is becoming general due to the fact that these materials allow great stiffness to be obtained with low mass, offering the airplane using these composite materials a performance surplus compared with an airplane made of metallic materials.
It has thus been proposed to use such composite materials to construct the main elements of fuselage primary structures, particularly the circumferential frames, the skins and the longitudinal stiffeners.
However, use of these composite materials does not allow these structural elements to deform plastically as elements made of metal do. On the contrary, when they are subjected to a large impact, these elements generally undergo explosive failure, resulting in a relatively low level of absorbed energy.
The use of primary fuselage structures of known type, the main elements whereof are made of composite materials, thus considerably reduces the quantity of energy that can be absorbed by the structure in a crash, particularly in the case of an impact located at the belly of the fuselage, vertically below the central longitudinal axis of that fuselage, which corresponds to the scenario generally used by certification authorities.